Due to relatively low cost and advancements in cellular radio technology, most metropolitan areas are now equipped with cellular telephone networks, similar to the system shown in FIG. 1.
The cellular telephone system 10 includes a mobile telephone switching office (MTSO) 12 and a plurality of cell site transceivers 14a-14c. The cell site transceivers transmit radio signals to and receive radio signals from one or more mobile units 16 that may move about a cellular service area 20. A mobile unit, as the term is used herein, refers to a cellular voice telephone or data receiver that can be permanently installed at a fixed location or within a vehicle or that can be portable. Each cell site transceiver 14 is able to broadcast and receive the radio signals within a geographic area 18 called the cell site coverage area. Together the areas 18 comprise the entire cellular service area 20. Typically, a cellular service area comprises a metropolitan area or larger region.
When a telephone call to a called mobile unit 16 originates from either another mobile unit or a land-based telephone via a public switched telephone network (PSTN) 25, a caller must first access the cellular telephone system 10. This is accomplished by dialing the mobile unit's unique identification number (i.e., its phone number). The MTSO 12 receives such a call request, and instructs a central call processor 17 to begin call processing. The central call processor 17 transmits a signal over a dedicated line 19 (such as a telephone line or microwave link, etc.) to each of the cell site transceivers 14a-14c causing the cell site transceivers to transmit a page signal that locates the mobile unit 16. The page signal alerts a particular mobile unit 16 that it is being called by including as part of the page signal the paged mobile unit's identification or phone number.
Each cell site transceiver 14 transmits the page signal on one or more dedicated forward control channels that carry all pages, as well as control signals, channel assignments, and other overhead messages to each mobile unit. The forward control channel is distinct from the voice channels that actually carry voice communications between a mobile unit and another mobile unit or a land-based telephone. Each cell site transceiver may have more than one forward control channel upon which pages can be carried. For clarity, however, this discussion will assume that each cell site transceiver has one forward control channel.
When a mobile unit is not engaged in a telephone call, it operates in an idle state. In the idle state, the mobile unit will tune to the strongest available forward control channel and monitor that channel for a page signal or other messages directed to it. Upon determining that a page signal is being transmitted, the mobile unit 16 again scans all forward control channels so as to select the cell site transceiver transmitting the strongest signal. The mobile unit then transmits an acknowledgment signal to the cell site transceiver over a reverse control channel associated with the strongest forward control channel. This acknowledgment signal serves to indicate to the MTSO 12 which of the several forward control channels (associated with the several cell site transceivers 14a-14c) to use for further call processing communications with mobile unit 16. This further communication typically includes a message sent to the mobile unit instructing it to tune to a particular voice channel for completion of call processing and for connection with the calling party. The details of how the cell site transceivers transmit the signals on the forward and reverse control channels are typically governed by standard protocols such as the EIA/TIA-553 Specification and the air interface standards for Narrow Band Analog Mobile Phone Service (NAMPS) and similar air interface standards for digital communications, all of which are well known to those of ordinary skill in the cellular telephone communications art and therefore will not be discussed.
While cellular phones have been quickly embraced by those people whose business requires them to travel frequently and to be in constant contact with their clients, many individuals are discouraged from utilizing present cellular telephone systems. The main reason for this discouragement is the relatively high price charged by the cellular service providers for a cellular telephone call. In conventional land-based telephone systems, the price of a telephone call can be tailored to reflect the supply and demand of the telephone resources available to a given area. For example, in large metropolitan areas or financial districts where telephone traffic and the cost of providing service is high, users can expect to pay more for a call than in a residential area where fewer calls are made. However, with current cellular technology, like the cellular telephone system shown in FIG. 1, the cellular service user has no way of knowing where in the cellular service area the user is. Therefore, cellular service providers have had difficulty creating services that require a user to know his location within the cellular service area.
Consequently, a need has developed to provide a system that allows a user to determine whether he is in a particular predefined zone within the cellular service area and to convey that information to the cellular service system. It is further desirable that such a system be compatible with existing cellular technology and should not degrade the operation of an existing cellular system. Finally, such a system should neither require the allocation of more radio frequencies than are currently allocated to cellular telephone systems, nor require a substantial portion of existing cellular frequencies.